A Guide to Making Multi-Slits for the COSMIC Spectrograph
and How to Use Them at the Telescope

Mike Pahre & Roy Gal

Updated: April 1998

This document also available as PostScript here or LaTeX here.

Preparing the Masks

The available field-of-view for COSMIC multi-slits is . It is suggested that the width of the multi-slit pattern be less than 4 arc-min, preferably even less than this, to ensure that the spectra have approximately the same central wavelength (and hence spectral coverage) and to avoid problems with spatial distortion. Since the multi-slit masks are made by a photographic reproduction process, the width of each slit can be set by the user.

The COSMIC slitmask assignment program takes a list of offsets (arc-sec East and arc-sec North, where East and North are positive) of objects from the center of a field. This program, called ``cosmicslitmask'', was written by Mike Pahre based on notes supplied by Alan Dressler. It is a simple program, assuming a constant plate scale (11.9''/mm) for the prime focus, and ignoring the complications of spatial distortion, gnomic projections, etc. The help file for this program is in anonymous ftp on phobos:/users/map/COSMIC/README (or can be accessed from phobos itself as ftp/users/map/COSMIC/README).

The output of this program consists of a text file and a plot file. You should create two separate plot files--one which includes the object number above each slit, the other with only the slits and the identification label. The latter plot is the one that needs to go to the photographic lab for reduction.

Note the field identification label on the upper left corner of the plot. The position angle for the mask in terms of the COSMIC ``base position angle'' is given in this label. The limits on rotation for the COSMIC base position angle is 70 to 300 degrees. (MAP put a software check into COSMICslitmask)

Make sure to take to the mountain printed copies of all 3 of these files (one text and two plots) for each mask you design. You should also have a good image of the field with all of the objects identified.

The slit masks used by the COSMIC spectrograph are made by reproducing photographically at a reduction factor of 4 the output of the COSMIC slit mask assignment program. Thus the slits correspond to places where the film is clear (but there is still some transmission loss of course), and the remaining area must be very opaque. If the contrast in transmission between the slit and the dark area is not at least 104, then sky light may well leak in and contaminate the exposure at a noticable level. Thus the two critical concerns for the photographic lab are the exact reduction by a factor of 4 of the supplied image and making the mask area (except for the slits) as opaque as possible.

Commercial Graphics (681 S. Raymond Ave., Pasadena, CA 91105) is the vendor of choice. They will have to send the materials to their LA Office to be photographed. The instructions to the vendor are to:

reduce 4 times
make a negative
reverse reading

Four masks are reproduced (reduced by a factor of 4) onto a sheet of 8.5 by 11 film.

Mounting the Masks into the Holders

There are 6 positions in the aperture wheel, position 1 of which must be left clear for direct images. The 1.5 arc-sec long slit is usually left in position 2, leaving 4 positions for slit-masks. There are 6 holders for slit masks.

To mount a photographic film slitmask onto a holder you will need: light table; scissors; narrow black tape (for blocking out the mask border); and a jeweler's screwdriver. All except for the black tape can be provided by the day crew. The tape should be bought in town; good examples are ``Letraline 476 3/16-inch black matt 3730'' or ``Letraline 96 1/4-inch black flex 2426''. Call around for a graphic design store to find such tape. You may also use the black photographic tape which is kept in the wooden desk just outside the control room, on the observing floor.

Using scissors, trim the slitmask film to keep  inch outside the rectangular mask border. Also, diagonally cut off the four corners of the film.

Take a mask holder and remove the cover (there should be four screws holding it in position). There are three notches cut into the outer ring of the holder. One notch is closest to a short side of the rectangular mask hole; place this notch facing towards your left. Turn your slitmask upside down so that the label reads backwards and lies in the upper right corner, and place on top of the mask holder. Place the cover on top, and lightly fix in place with the screws, leaving it loose enough so that you can still move the film around.

Place the mask holder on the light table. Slide the film around until the upper/lower mask border lines up with the upper/lower edge of the rectangular cutout of the mask holder. This is a crucial step to do carefully, as it sets the rotation angle of the slits in the mask. When you are satisfied, tighten the four screws to fasten the film in place.

Use the black tape to cover the four clear borders of the film and the label. Also cover any imperfections in the black portion of the emulsion.

Be sure to note on the tape the identification or name of the mask.

Inserting the Slit Masks into the Spectrograph

As you enter the prime focus cage of the 200-inch telescope, move around counter-clockwise to provide easy access to the blue box (the COSMIC local motor control console). Assuming the zipper on the COSMIC dark shroud is in its usual place, you also then will have easy access to a velcro-closed opening in the shroud at about waist level. Open this to get access to the aperture wheel. The first time you try this you might want to unzip the zipper on the COSMIC dark shroud, so that you can see the entire instrument more easily. (Sometimes the setup crew leaves this shroud in a position rotated from its correction orientation, so you might have to remove the shroud anyway.)

Use the blue box to move the desired aperture wheel position to the install location. First push ``AP'' to select the aperture motor, then push ``INC'' or ``DEC'' to move the wheel. The position displayed by the blue box is the position of the aperture wheel that is in the optical path; the position in the installation location at that time is 2 positions less (modulo 6).

Reach over the blue motor housing mounted just above the aperture wheel on COSMIC. Hold the holder so the ``locating notch'' is pointing away from the blue motor housing. Insert the holder and mask so that the 3 notches all clear, then press down, and turn clockwise approximately 1/4 inch, after which you will hit a stop. The stop is defined by knife blades hitting the beveled edge of the each of holder notches. (CHECK ALL KNIFE BLADES ARE INSTALLED). For peace of mind, tape the holder to the aperture wheel once the holder is installed.

Repeat this procedure using the blue box to move to each desired position on the aperture wheel for each mask you plan to use.

Be sure to close the velcro flap on the shroud prior to leaving the prime focus cage.

Once you are back in the control room, cycle through several different positions on the aperture wheel to reset it properly.

Preparatory Work in the Afternoon

You should write a tape at your home institution including the tabular output files from cosmicslitmask.f. Dump these files into your current COSMIC directory on the COSMIC Sparc computer (i.e. where the data are being written).

For each mask you need to create a file containing your alignment objects. This should be done in the afternoon before observing, and the file should be in the same directory that you are using for taking images. This can be done easily by editing the tabular output of the cosmicslitmask.f program: (1) remove the header line(s); (2) keep only 5-10 objects that are good for slitmask alignment; and (3) reorder them so that the objects go from left to right (in the plot of the slitmask). A good object for slitmask alignment is relatively bright and compact, so that an accurate centroid can be measured in a relatively short imaging exposure. Call this file ``mask.slits''.

Take a direct image of each of the slit-masks you have just installed. This images should be taken with the FILTER=CLEAR, GRISM=CLEAR, and APERTURE=multislit of your choice.

Display the image of the desired slit-mask (with the [*,-*] orientation). Once you have taken the images, use the IRAF script ``masksetup'' to mark the positions of the slits corresponding to the objects in the ``mask.slits'' file. In IRAF, type ``masksetup''. You will be asked for an output filename (typically ``mask'') and the input filename (typically ``mask.slits''). You will then be asked to press the ``b'' key at the left and right edges of the slit corresponding to your first object, followed by ``q'', then the ``a'' key at the center of slit, followed by ``q''. This will be repeated for each alignment slit. After the last slit, the calculated positions of the objects will be marked on the screen, and an output file called ``masks.ap'' will be produced which will be used during the night for alignment.

Aligning the Slit-Mask on a Field on the Sky

The IRAF script MASKALIGNOFF is used to align your masks. Set the COSMIC base position angle to the correct value, which is indicated in the slitmask plot label. First, take a short (30s) exposure of the field you will be using your mask on. Do not use any filter (filter position 1, clear), and leave the aperture (aperture position 1, clear) and grism (either position 2 or 4) wheels open.

When the image has read out, display it (with the [*,-*] orientation). Execute MASKALIGNOFF, which will prompt you for the input file with slit positions (``mask.ap'', but without the .ap), and the image to work on. This program will then draw circles around the positions corresponding to the desired positions (in the slitmask) of the objects you have identified. This may be many arcseconds away from their actual positions in the image! Move to the objects in the order they were entered into the file, and at each one, put the cursor on the object, then hit the spacebar. After the last object, type ``q'' (for ``quit''). (You will need to ``quit'' twice, first in the display window, then in the IRAF window.)

This produces an output in the IRAF window of the mean offset for X and Y arc-sec and for theta in degrees, as well as errors from the mean fit in X and Y for each object. For a properly made and mounted mask, the errors of the objects with respect to the fit will be small, under 1.5 arc-sec, even when you are quite far off in pointing and/or rotation angle. If you see larger errors, either the coordinate list fed into the mask design program is incorrect or there has been a mistake in how the mask was mounted. If you see large errors of alternating sign, the most likely flaw is that the mask has been loaded upside down, and needs to be flipped with respect to the holder.

You then be prompted to type in the COSMIC base position angle, so the offets in X and Y can be rotated into offsets in RA and Dec. Move the telescope as required. If the offset dRA is positive, move East. If dDEC is positive, move the telescope North. Add dTHETA to the COSMIC base position angle and rotate COSMIC as required. If the required rotation angle is less than 0.1,then you should not adjust it.

Two iterations will probably suffice for most fields given reasonable centerfield coordinates and telescope pointing. You should obtain a guide star after the first iteration (i.e. after the first telescope offset, and before starting the second image exposure), as this is an integral part of the final alignment. If it is necessary to make small moves after the first iteration, the guider can be placed in ``follow'' mode, the telescop moved, and guiding restarted.

You should now insert the appropriate mask and grism, and start taking your spectra. The entire alignment. procedure should take about 12 minutes (6 of which are readout).

Other Comments

If you wish to sample a wavelength range that is not centered on the standard wavelength of the grism, this can be accomplished by moving all of the slits up or down (in the y-direction) in the slitmask field-of-view. This is typically not a reasonable course of action for the 300 l/mm grism, since its spectral coverage includes nearly the entire optical spectrum. But it may be of interest for users of the 600 l/mm grating (central wavelength is 5000 Å). For the latter grism, the spectral scale of 1.6 Åper pixel, the field-of-view in the y-direction of , and the scale of 0.4'' per pixel, result in an available range of  Åfor the central wavelength. (With this grism, the full spectral coverage is approximately 3200 Å.) You will need to avoid the label in the upper-left portion of the mask, and should be advised that any distortions in the COSMIC field-of-view will be more pronounced as the slits are placed further from the center of the field. Moving the slits upwards (in the positive y-direction) will shift the center wavelength towards the red. This can be done in the ``cosmicslitmask'' program (option 10) which you use to create the slitmasks.

The scale of your input object coordinates should be known quite accurately; using a ``word-of-mouth'' CCD pixel scale, or one listed in a handbook, might not be as accurate as you might think. The best solution is to observe a globular cluster with the same CCD setup as you observed your multislit object field, and then use astrometry programs to get a good plate scale. Better yet, you can use many stars (> 100) from the cluster to get a mapping of the CCD distortion, and correct your object coordinate list for this effect. For illustration purposes, a 0.5% error in the plate scale will cause a shift in the slits by 1.2'' for a 4' wide set of multislits, which is comparable to the typical slit width of 1.5''. Accuracy of 0.2% or better in the plate scale is a reasonable goal.

The focusing of the CCD camera on the apertures is dependent on temperature. This adjustment is not typically done by the setup crew, but should be considered by the observer. Measure the slit widths (FWHM) in the afternoon and determine if they appear to be in an acceptable focus. The focus procedure requires two people, one in the cage turning screws and the other at the computer taking images and measuring FWHMs. Contact experienced individuals before attempting this procedure, as mistakes can be destructive to the alignment of the instrument. It is also important to note that the optimal focus for the longslit and for your multislits might be different, so you may have to choose which one to optimize.

Roy Gal